1. Field of the Invention
The present invention relates generally to the field of line terminating devices, and, more particularly, the present invention relates to an improved CMOS line termination circuit.
2. Description of the Related Art
In communication systems and computers, lines having a defined characteristic impedance must be used for transmission of data at extremely high rates and the end of the line must be terminated with a resistor that corresponds to the characteristic impedance of the line. Failure to provide such a termination results in reflections and line disturbances that interfere with the desired signal and result in data errors.
Data rates in the range from approximately 100 MBit/s through 1 GBit/s were previously reserved for bipolar or gallium arsenide components in integrated circuits. However, as a result of progress in semiconductor technology, CMOS modules have also now reached this speed range. Terminating resistors are therefore also required at the inputs of CMOS modules for transmission of such high data rates.
Up to now, the terminating resistors of CMOS modules have been located outside the module on the printed circuit board close to the corresponding input pins. Modules of switching systems, for example switching network modules, or computer memories having address and data lines, have a large number of inputs so that substantial space is required for the corresponding terminating resistors. This reduces the packing density on the assembly. Additionally, there is a loss of performance when used with high frequency signals because the termination is not directly connected to the input stage of the receiver module. The housing of the module as well as electrostatic charge protection structure which are required still in CMOS modules are located between terminating resistor and input stage. When viewed electrically, the housing is essentially composed of line inductances and capacitances, such as pin and bond inductances and parasitic capacitors. Additionally there are coupling inductances and capacitances between the pins. The path from the external terminating resistor to the input stage can be viewed as a non-terminated line which results in reflections and consequently the transmission of high frequency signals is difficult. Terminating resistors in IC modules are known in the art, such as for example, Safferthal, "Terminieren von Signalleitungen", Elektronik 22/1990, pp. 236-254.
It is therefore desirable to terminate the line on the chip directly at the input stage. In standard CMOS technology, a sufficiently precise resistor layer is not currently available. Polysilicon or metal tracks and well resistors have manufacturing tolerances and temperature coefficients that are too high. These potentially lead to total variations of .+-.50% in the resistance value which is unacceptable. With such variations, undesired reflections and large fluctuations in the input levels (when the transmitter operates as current source, which is often the case) arise. A precision MOS resistor together with the corresponding drive circuit is disclosed, for example by GB-A-2 248 143.